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<!DOCTYPE refentry [ <!ENTITY % mathent SYSTEM "math.ent"> %mathent; ]>
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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glMemoryBarrier">
<info>
<copyright>
<year>2011-2014</year>
<holder>Khronos Group</holder>
</copyright>
</info>
<refmeta>
<refentrytitle>glMemoryBarrier</refentrytitle>
<manvolnum>3G</manvolnum>
</refmeta>
<refnamediv>
<refname>glMemoryBarrier</refname>
<refpurpose>defines a barrier ordering memory transactions</refpurpose>
</refnamediv>
<refsynopsisdiv><title>C Specification</title>
<funcsynopsis>
<funcprototype>
<funcdef>void <function>glMemoryBarrier</function></funcdef>
<paramdef>GLbitfield <parameter>barriers</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>void <function>glMemoryBarrierByRegion</function></funcdef>
<paramdef>GLbitfield <parameter>barriers</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1 xml:id="parameters"><title>Parameters</title>
<variablelist>
<varlistentry>
<term><parameter>barriers</parameter></term>
<listitem>
<para>
Specifies the barriers to insert.
</para>
<para>
For <function>glMemoryBarrier</function>, must be a
bitwise combination of any of
<constant>GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT</constant>,
<constant>GL_ELEMENT_ARRAY_BARRIER_BIT</constant>,
<constant>GL_UNIFORM_BARRIER_BIT</constant>,
<constant>GL_TEXTURE_FETCH_BARRIER_BIT</constant>,
<constant>GL_SHADER_IMAGE_ACCESS_BARRIER_BIT</constant>,
<constant>GL_COMMAND_BARRIER_BIT</constant>,
<constant>GL_PIXEL_BUFFER_BARRIER_BIT</constant>,
<constant>GL_TEXTURE_UPDATE_BARRIER_BIT</constant>,
<constant>GL_BUFFER_UPDATE_BARRIER_BIT</constant>,
<constant>GL_FRAMEBUFFER_BARRIER_BIT</constant>,
<constant>GL_TRANSFORM_FEEDBACK_BARRIER_BIT</constant>,
<constant>GL_ATOMIC_COUNTER_BARRIER_BIT</constant>, or
<constant>GL_SHADER_STORAGE_BARRIER_BIT</constant>.
</para>
<para>
For <function>glMemoryBarrier</function>, must be a
bitwise combination of any of
<constant>GL_ATOMIC_COUNTER_BARRIER_BIT</constant>, or
<constant>GL_FRAMEBUFFER_BARRIER_BIT</constant>,
<constant>GL_SHADER_IMAGE_ACCESS_BARRIER_BIT</constant>,
<constant>GL_SHADER_STORAGE_BARRIER_BIT</constant>.
<constant>GL_TEXTURE_FETCH_BARRIER_BIT</constant>, or
<constant>GL_UNIFORM_BARRIER_BIT</constant>.
</para>
<para>
If the special value
<constant>GL_ALL_BARRIER_BITS</constant> is specified,
all supported barriers for the corresponding command
will be inserted.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1 xml:id="description"><title>Description for <function>glMemoryBarrier</function></title>
<para>
<function>glMemoryBarrier</function> defines a barrier ordering
the memory transactions issued prior to the command relative to
those issued after the barrier. For the purposes of this
ordering, memory transactions performed by shaders are
considered to be issued by the rendering command that triggered
the execution of the shader. <parameter>barriers</parameter> is
a bitfield indicating the set of operations that are
synchronized with shader stores; the bits used in
<parameter>barriers</parameter> are as follows:
</para>
<para>
<variablelist>
<varlistentry>
<term><constant>GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT</constant></term>
<listitem>
<para>
If set, vertex data sourced from
buffer objects after the barrier will reflect data written by shaders
prior to the barrier. The set of buffer objects affected by this bit
is derived from the buffer object bindings used for
generic vertex attributes derived from the <constant>GL_VERTEX_ATTRIB_ARRAY_BUFFER</constant> bindings.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_ELEMENT_ARRAY_BARRIER_BIT</constant></term>
<listitem>
<para>
If set, vertex array indices sourced from
buffer objects after the barrier will reflect data written by shaders
prior to the barrier. The buffer objects affected by this bit are
derived from the <constant>GL_ELEMENT_ARRAY_BUFFER</constant> binding.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_UNIFORM_BARRIER_BIT</constant></term>
<listitem>
<para>
Shader uniforms sourced from buffer objects after the barrier will reflect data
written by shaders prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_TEXTURE_FETCH_BARRIER_BIT</constant></term>
<listitem>
<para>
Texture fetches from shaders, including
fetches from buffer object memory via buffer textures, after the
barrier will reflect data written by shaders prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_SHADER_IMAGE_ACCESS_BARRIER_BIT</constant></term>
<listitem>
<para>
Memory accesses using shader image
load, store, and atomic built-in functions issued after the barrier
will reflect data written by shaders prior to the barrier.
Additionally, image stores and atomics issued after the barrier will
not execute until all memory accesses (e.g., loads, stores, texture
fetches, vertex fetches) initiated prior to the barrier complete.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_COMMAND_BARRIER_BIT</constant></term>
<listitem>
<para>
Command data sourced from buffer objects by
Draw*Indirect commands after the barrier will reflect data written by
shaders prior to the barrier. The buffer objects affected by this bit
are derived from the <constant>GL_DRAW_INDIRECT_BUFFER</constant> binding.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_PIXEL_BUFFER_BARRIER_BIT</constant></term>
<listitem>
<para>
Reads and writes of buffer objects via the
<constant>GL_PIXEL_PACK_BUFFER</constant> and <constant>GL_PIXEL_UNPACK_BUFFER</constant>
bindings (via <citerefentry><refentrytitle>glReadPixels</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glTexSubImage</refentrytitle></citerefentry>, etc.) after the
barrier will reflect data written by shaders prior to the barrier.
Additionally, buffer object writes issued after the barrier will wait
on the completion of all shader writes initiated prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_TEXTURE_UPDATE_BARRIER_BIT</constant></term>
<listitem>
<para>
Writes to a texture via <function>glTex(Sub)Image*</function>,
<function>glCopyTex(Sub)Image*</function>, <function>glCompressedTex(Sub)Image*</function>, and reads via
<citerefentry><refentrytitle>glGetTexImage</refentrytitle></citerefentry> after the barrier will reflect data written by shaders
prior to the barrier. Additionally, texture writes from these
commands issued after the barrier will not execute until all shader
writes initiated prior to the barrier complete.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_BUFFER_UPDATE_BARRIER_BIT</constant></term>
<listitem>
<para>
Reads or writes via <citerefentry><refentrytitle>glBufferSubData</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glCopyBufferSubData</refentrytitle></citerefentry>,
or <citerefentry><refentrytitle>glGetBufferSubData</refentrytitle></citerefentry>, or
to buffer object memory mapped by <citerefentry><refentrytitle>glMapBuffer</refentrytitle></citerefentry>
or <citerefentry><refentrytitle>glMapBufferRange</refentrytitle></citerefentry> after the barrier
will reflect data written by shaders prior to the barrier.
Additionally, writes via these commands issued after the barrier will
wait on the completion of any shader writes to the same memory
initiated prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_FRAMEBUFFER_BARRIER_BIT</constant></term>
<listitem>
<para>
Reads and writes via framebuffer object
attachments after the barrier will reflect data written by shaders
prior to the barrier. Additionally, framebuffer writes issued after
the barrier will wait on the completion of all shader writes issued
prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_TRANSFORM_FEEDBACK_BARRIER_BIT</constant></term>
<listitem>
<para>
Writes via transform feedback
bindings after the barrier will reflect data written by shaders prior
to the barrier. Additionally, transform feedback writes issued after
the barrier will wait on the completion of all shader writes issued
prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_ATOMIC_COUNTER_BARRIER_BIT</constant></term>
<listitem>
<para>
Accesses to atomic counters after the
barrier will reflect writes prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_SHADER_STORAGE_BARRIER_BIT</constant></term>
<listitem>
<para>
Accesses to shader storage blocks after the
barrier will reflect writes prior to the barrier.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><constant>GL_QUERY_BUFFER_BARRIER_BIT</constant></term>
<listitem>
<para>
Writes of buffer objects via the <constant>GL_QUERY_BUFFER</constant>
binding after the barrier will reflect data written
by shaders prior to the barrier. Additionally, buffer object writes
issued after the barrier will wait on the completion of all shader
writes initiated prior to the barrier.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
If <parameter>barriers</parameter> is
<constant>GL_ALL_BARRIER_BITS</constant>, shader memory accesses
will be synchronized relative to all the operations described
above.
</para>
<para>
Implementations may cache buffer object and texture image memory that
could be written by shaders in multiple caches; for example, there may be
separate caches for texture, vertex fetching, and one or more caches for
shader memory accesses. Implementations are not required to keep these
caches coherent with shader memory writes. Stores issued by one
invocation may not be immediately observable by other pipeline stages or
other shader invocations because the value stored may remain in a cache
local to the processor executing the store, or because data overwritten by
the store is still in a cache elsewhere in the system. When
<function>glMemoryBarrier</function> is called, the GL flushes
and/or invalidates any caches relevant to the operations
specified by the <parameter>barriers</parameter> parameter to
ensure consistent ordering of operations across the barrier.
</para>
<para>
To allow for independent shader invocations to communicate by reads and
writes to a common memory address, image variables in the OpenGL Shading
Language may be declared as "coherent". Buffer object or texture image
memory accessed through such variables may be cached only if caches are
automatically updated due to stores issued by any other shader invocation.
If the same address is accessed using both coherent and non-coherent
variables, the accesses using variables declared as coherent will observe
the results stored using coherent variables in other invocations. Using
variables declared as "coherent" guarantees only that the results of
stores will be immediately visible to shader invocations using
similarly-declared variables; calling
<function>glMemoryBarrier</function> is required to ensure that
the stores are visible to other operations.
</para>
<para>
The following guidelines may be helpful in choosing when to use coherent
memory accesses and when to use barriers.
</para>
<para>
<itemizedlist>
<listitem>
<para>
Data that are read-only or constant may be accessed
without using coherent variables or calling
<function>MemoryBarrier</function>(). Updates to the
read-only data via API calls such as
<function>glBufferSubData</function> will invalidate
shader caches implicitly as required.
</para>
</listitem>
<listitem>
<para>
Data that are shared between shader invocations at a
fine granularity (e.g., written by one invocation,
consumed by another invocation) should use coherent
variables to read and write the shared data.
</para>
</listitem>
<listitem>
<para>
Data written by one shader invocation and consumed
by other shader invocations launched as a result of
its execution ("dependent invocations")
should use coherent variables in the producing
shader invocation and call
<function>memoryBarrier</function>() after the last
write. The consuming shader invocation should also
use coherent variables.
</para>
</listitem>
<listitem>
<para>
Data written to image variables in one rendering
pass and read by the shader in a later pass need not
use coherent variables or memoryBarrier(). Calling
<function>glMemoryBarrier</function> with the
SHADER_IMAGE_ACCESS_BARRIER_BIT set in
<parameter>barriers</parameter> between passes is
necessary.
</para>
</listitem>
<listitem>
<para>
Data written by the shader in one rendering pass and
read by another mechanism (e.g., vertex or index
buffer pulling) in a later pass need not use
coherent variables or
<function>memoryBarrier</function>(). Calling
<function>glMemoryBarrier</function> with the
appropriate bits set in
<parameter>barriers</parameter> between passes is
necessary.
</para>
</listitem>
</itemizedlist>
</para>
</refsect1>
<refsect1 xml:id="description2"><title>Description for <function>glMemoryBarrierByRegion</function></title>
<para>
<function>glMemoryBarrierByRegion</function> behaves as
described above for <function>glMemoryBarrier</function>, with
two differences:
</para>
<para>
First, it narrows the region under consideration so that only
reads and writes of prior fragment shaders that are invoked for
a smaller region of the framebuffer will be completed/reflected
prior to subsequent reads and writes of following fragment
shaders. The size of the region is implementation-dependent and
may be as small as one framebuffer pixel.
</para>
<para>
Second, it only applies to memory transactions that may be read
by or written by a fragment shader. Therefore, only the barrier
bits
<itemizedlist>
<listitem>
<para><constant>GL_ATOMIC_COUNTER_BARRIER_BIT</constant></para>
</listitem>
<listitem>
<para><constant>GL_FRAMEBUFFER_BARRIER_BIT</constant></para>
</listitem>
<listitem>
<para><constant>GL_SHADER_IMAGE_ACCESS_BARRIER_BIT</constant></para>
</listitem>
<listitem>
<para><constant>GL_SHADER_STORAGE_BARRIER_BIT</constant></para>
</listitem>
<listitem>
<para><constant>GL_TEXTURE_FETCH_BARRIER_BIT</constant></para>
</listitem>
<listitem>
<para><constant>GL_UNIFORM_BARRIER_BIT</constant></para>
</listitem>
</itemizedlist>
are supported.
</para>
<para>
When <parameter>barriers</parameter> is
<constant>GL_ALL_BARRIER_BITS</constant>, shader memory accesses
will be synchronized relative to all these barrier bits, but not
to other barrier bits specific to
<function>glMemoryBarrier</function>. This implies that reads
and writes for scatter/gather-like algorithms may or may not be
completed/reflected after a
<function>glMemoryBarrierByRegion</function> command. However,
for uses such as deferred shading, where a linked list of
visible surfaces with the head at a framebuffer address may be
constructed, and the entirety of the list is only dependent on
previous executions at that framebuffer address,
<function>glMemoryBarrierByRegion</function> may be
significantly more efficient than
<function>glMemoryBarrier</function>.
</para>
</refsect1>
<refsect1 xml:id="notes"><title>Notes</title>
<para>
<constant>GL_SHADER_STORAGE_BARRIER_BIT</constant> is available only if the GL version is 4.3 or higher.
</para>
<para>
<constant>GL_QUERY_BUFFER_BARRIER_BIT</constant> is available only if the GL version is 4.4 or higher.
</para>
</refsect1>
<refsect1 xml:id="errors"><title>Errors</title>
<para>
<constant>GL_INVALID_VALUE</constant> is generated if
<parameter>barriers</parameter> is not the special value
<constant>GL_ALL_BARRIER_BITS</constant>, and has any bits set
other than those described above for
<function>glMemoryBarrier</function> or
<function>glMemoryBarrierByRegion</function> respectively.
</para>
</refsect1>
<refsect1 xml:id="versions"><title>Version Support</title>
<informaltable>
<tgroup cols="13" align="left">
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="apifunchead.xml" xpointer="xpointer(/*/*)"/>
<tbody>
<row>
<entry><function>glMemoryBarrier</function></entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="apiversion.xml" xpointer="xpointer(/*/*[@role='42']/*)"/>
</row>
<row>
<entry><function>glMemoryBarrierByRegion</function></entry>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="apiversion.xml" xpointer="xpointer(/*/*[@role='45']/*)"/>
</row>
</tbody>
</tgroup>
</informaltable>
</refsect1>
<refsect1 xml:id="seealso"><title>See Also</title>
<para>
<citerefentry><refentrytitle>glBindImageTexture</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glBufferData</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glMapBuffer</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glMapBufferRange</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>glFlushMappedBufferRange</refentrytitle></citerefentry>,
<citerefentry><refentrytitle>memoryBarrier</refentrytitle></citerefentry>
</para>
</refsect1>
<refsect1 xml:id="Copyright"><title>Copyright</title>
<para>
Copyright <trademark class="copyright"/> 2011-2014 Khronos Group.
This material may be distributed subject to the terms and conditions set forth in
the Open Publication License, v 1.0, 8 June 1999.
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://opencontent.org/openpub/">http://opencontent.org/openpub/</link>.
</para>
</refsect1>
</refentry>
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